This invention relates to catalyst systems useful for polymerizing alpha-olefins and particularly relates to a supported titanium halide catalyst component and a catalyst system containing such component.
Magnesium-containing supported titanium halide-based alpha-olefin polymerization catalyst components are now well known in the art. Typically, these catalysts are recognized for their performance based on activity and stereospecificity. However, commercial olefin polymerization, especially gas-phase alpha-olefin polymerization, requires additional catalyst attributes for economical large-scale operation. Specifically, polymer morphology, typically dependent upon catalyst morphology, many times is critical. Included in good polymer morphology is uniformity of particle size and shape, resistance to attrition and an acceptably high bulk density. Minimization of very small particles (fines) typically is very important, especially in gas-phase polymerization to avoid transfer or recycle line pluggage. Very large particles also must be avoided to minimize formation of lumps and strings in the reactor.
A suitable catalyst should produce a polymer with minimum particles having average diameter less than 150 microns (fines) or greater than 850 microns. Another property which is important commercially is maintenance of an acceptably high bulk density. Typically, this property is measured in pounds per cubic foot of polymer. One approach to improve morphology control as described in U.S. Pat. No. 4,540,679 is to solubilize a magnesium species with carbon dioxide in an alcohol solvent. Although Example VI of this patent describes a high activity catalyst, its morphology needs improvement in that the catalyst produced too many fines.
The present invention incorporates a series of catalyst formation process steps which produce a catalyst with exceptionally high activity and stereospecificity combined with very good morphology.
Supported, magnesium-containing, titanium-containing, electron donor-containing olefin polymerization catalysts now are well known in the art. Use of carbon dioxide in preparing a magnesium-containing support has been described in the aforementioned U.S. Pat. No. 4,540,679. Also, in a different context use of carbon dioxide was described in U.S. Pat. No. 4,246,383; 2,244,838; 4,529,715; and 4,530,915. Treating alcoholic solutions and suspensions of alkaline earth alcoholates with carbon dioxide and sulfur dioxide was noted in Chemical Abstracts, Vol. 76, 853050v (1972). Forming a soluble magnesium species has been described in U.S. Pat. Nos. 4,315,874; 4,399,054; 4,071,674; and 4,439,540. Examples of use of silicon compounds in formation of a catalyst component include U.S. Pat. Nos. 4,071,672; 4,085,276; 4,220,554; and 4,315,835. Tetrahydrofuran (THF) has been described variously to complex a magnesium chloride species (e.g., U.S. Pat. Nos. 4,482,687, 4,277,372, 3,642,746, and 3,642,772 and in European Patent 131,832); as a modifier in a cocatalyst (e.g., U.S. Pat. Nos. 4,158,642 and 4,148,756); and as a solvent (e.g., U.S. Pat. Nos. 4,477,639 and 4,518,706 ). However the specific combination of steps taught in this invention to produce a catalyst with extremely advantageous properties have not been disclosed.